So for my Minako cosplay, I'm thinking about making the masks that the bosses in the game have. The problem is that most of them are weird, odd shapes and I'm not sure how to go about getting the different designs. I found a place online that sold a bunch of masks in different shapes and such(nothing very close to what I need but it is something). Any tips on getting them to the shapes I need? I'm guessing I'm going to need a base and build up from there? Like with craft foam?

Getting it to look natural by building onto one of those masks with craft foam would be difficult since it can't really blend in too well.

However, since they are made from paper pulp, you could easily extend them with paper mache and paper clay (celluclay), since they are pretty much the same material. You'd want to temporarily reinforce the structure by filling the underside with oil-based or polymer based clay. That way it won't go floppy when wet.